1. Field of the Invention
The invention relates to the separation of liquid mixtures, including solutions. More particularly, it relates to the separation of a mixture of liquids of differing volatilities by stage-wise or fractional distillation. Specifically, the present invention is directed to an improved method and apparatus for separating a liquid mixture by fractional distillation in a plate-type apparatus employing at least one plate consisting at least in part of a porous structure having capillary-type passages which permit of the capillary suction flow of liquid but not of the passage of vapor.
2. Description of Prior Art
Methods and apparatus for separating mixtures of volatile liquids are well known in the art. One early technique, i.e., "multiple distillation", involved the use of a series of separate stills. Another conventional technique, namely, "rectification", is an outgrowth of "multiple distillation", and resembles multiple distillation in its object and result but not necessarily in the mechanism by which it achieves the result.
In rectification, there is a direct interchange of the heat of condensation and the heat of evaporation at each stage of a multiple-stage operation conducted in a single apparatus. That apparatus, called a rectifying column, consists of a vertical column connected at the bottom to the outlet of a still, and divided into vertically-disposed compartments by a plurality of transverse plates. These plates have openings, such as risers, perforations or the like, which permit of the passage of vapor ascending from below, and devices such as bubble caps or the like which attempt to bring such vapor into intimate contact with the liquid maintained to a finite level on the plate's top surface. Each plate also has an overflow pipe, or downcomer, which maintains the desired, finite level of the liquid on the plate, and discharges excess liquid into the pool of liquid similarly maintained on the plate below at a point below the surface of such pool. The pool of liquid on each plate is prevented from passing downwardly through the openings on the plate either by the risers or, in the case of a perforated plate, by the vapor which is coming up through those perforations from the compartment below.
Each of the plates in a rectifying column functions more or less like a small still, in which the source of the heat of vaporization is the hot vapor coming from the plate below, and the cooling medium is the cooler liquid coming from the plate above. In theory, optimum enrichment of the ascending vapor by the descending liquid is achieved when the molal ratio of such liquid to such vapor is practically unity. In actual practice, however, a ratio of less than unity is maintained in order to reduce heat consumption. Consequently, the rate of enrichment of the ascending vapor by the descending liquid typically is not as rapid as would be expected in equimolal countercurrent flow.
The analogy between a rectifying column and a series of separate stills does not hold if the vapor leaving the liquid on a plate is not in equilibrium with that liquid. In all conventional rectifying columns, it has been found that the vapor above each plate contains less of the more volatile component or components than would be the case if complete equilibrium had been reached. This departure from equilibrium between the rising bubbles of vapor and the surrounding liquid on a plate is the driving force for a molecular and turbulent transport of mass and heat between the vapor and the liquid, the rates of which transport processes vary directly with interfacial area, concentration and temperature differentials between the two phases, turbulence within the phases, and contacting time. The bubbling action in a conventional rectifying column also causes entrainment of liquid in the vapor, resulting in the back-mixing of the descending liquid coming from the plate above.
As is apparent from the above description, the rate and efficiency of liquid fractionation by conventional rectification are controlled by the mechanisms and rates of mass and heat transfer between the two fluid phases, and are limited by liquid entrainment. However, because of the complexity of the mechanisms of these two transfer processes, and the fact that, historically, rectification is an outgrowth of multiple distillation, in practice rectification has been regarded and treated as an equilibrium process rather than as a rate process. This attitude is reflected in the universal practice of determining the so-called "plate efficiency" for a conventional rectification plate in terms of the approach to equilibrium expressed as a fraction or per cent.
Since the turn of this century, many workers in the field have measured the plate efficiencies for various liquid systems, plate designs and operating conditions, and have attempted to improve plate efficiencies through changes in the geometrical design of the plates and in operating conditions. Plate efficiencies for known plate designs have varied widely from about 10 percent to about 90 percent, depending upon the liquid system, plate design and operating conditions. The average efficiency for plate designs commonly employed in conventional rectifying columns used in industry is less than 65 percent. No known plate design has been found to provide a plate efficiency of 100 percent.
In view of the above, a principal object of the present invention is to provide a novel liquid fractionation method and apparatus which avoid the difficulties normally associated with conventional rectification. In particular, it is an object of the present invention to provide a fractionation method and apparatus which permit of the elimination of vapor flow through the plates, and the consequent intermixing between the two fluid phases, which typically occur in conventional rectification.
Another and primary object of the present invention is to provide a novel method and apparatus for separating mixtures of volatile liquids by fractional distillation in accordance with a mechanism which is totally different from that of conventional rectification. Thus, it is an object of the present invention to provide a fractionation method and apparatus which employ plates having capillary-type passages adapted to effect condensation of vapor mixtures and transfer and subsequent equilibrium evaporation of the condensed liquid under the influence of capillary pressures.
A further object of the present invention is to provide a novel plate design for liquid fractionating columns, through the use of which extremely high efficiencies, even approaching or exceeding 100 percent, are obtained.
Another important object of the present invention is to provide a novel method and apparatus which are particularly effective for separating mixtures of liquids whose respective volatilities are relatively close.
A further object of the invention is to provide a novel fractionation apparatus whose operation characteristically involves an uncommonly low pressure drop.
Still another object of the invention is to provide a novel fractionation apparatus employing plates adapted to effect a rapid transfer of heat from the condensing vapor beneath the plate to the liquid above the plate, and rapid and equilibrium vaporization of the liquid on the plate through nucleate boiling over the surface of the plate.